Traditionally Light Emitting Diodes (LEDs) have primarily been used as indicator lamps in electronic equipment. However recently the power and efficacy (e.g., lumens per watt of electrical power) has been increasing and LEDs have been identified as a possible replacement for inefficient incandescent lamps in certain applications. The light emitting region of an LED is small (e.g., in the range of 2 mm to 0.7 mm across in many cases) which in theory opens up the possibility for highly controlled distribution of light. However many of LED optics developed so far do not produce controlled distributions, rather they typically produce Gaussian like distributions which is the hallmark of somewhat uncontrolled (random) light distribution, and is not ideal for most, if not all applications.
A bare LED chip or an LED chip covered in an encapsulating protective transparent hemisphere, emits light over an entire hemisphere of solid angle, albeit with diminishing intensity at polar angles (zenith angles) approaching π/2. FIG. 1 is a plot light intensity in arbitrary units as a function of polar angle for a commercial high power LED.
FIG. 2 shows a reflector 202 arranged to collect a portion of light emitted by an LED 204. A problem with using a reflector with an LED that emits over the entire hemisphere of solid angle is that the reflector needs to have an aperture and thus cannot intercept and redirect all of the light. As shown in FIG. 1 light emitted within polar angle range from zero to φ passes through the aperture of the reflector 202 without redirection or control. Additionally for the reflector 202 to exert detailed control over the emitted light distribution it must be specular as opposed to diffuse, and polishing a reflector sufficiently to make it specular is often expensive.
In an attempt to address the problem posed by the hemispherical range of light output from LED, a type of “primary” optic 302 shown in FIG. 3 has been developed. (This is termed a “primary” optic because it is assumed that it may be used in conjunction with a “secondary” optic such as the reflector 202.) The term “primary optic” may also be taken to mean an optic which has an optical medium of index >1 extending from the LED die so that there are only outer optical surfaces. The primary optic 302 is designed to intercept light emitted by an LED chip which is positioned in a space 304 at the bottom of the primary optic 302 and to redirect the light radially outward, perpendicular to an optical axis 306. The primary optic includes a refracting part 308 and a TIR (Total Internal Reflection) part 310 both of which contribute to redirecting the light. One drawback of the primary optic 302 is that because it includes multiple optical surfaces that contribute to light in the same direction it will increase the effective size of the source (also the étendue), which reduces the controllability of light from the LED. The increased effective size of the source can in some cases be compensated for, by using larger secondary optics but this may be undesirable based on cost and space constraints. By way of loose analogy to imaging optics, the primary optic creates multiple “images” of the LED, e.g., one from the refracting part 308 and one from the TIR part 310.
Although, the primary optic 302 is intended to redirect light perpendicular to the optical axis, in practice light is redirected to a range of angles. This is because the primary optic is small and positioned in close proximity to the LED, and consequently the LED subtends a not-insignificant solid angle from each point of the primary optic, and light received within this finite solid angle is refracted or reflected into a commensurate solid angle. The result is shown in FIG. 4 which is a plot of light intensity vs. polar angle for an LED equipped with the primary optic 302. Although this distribution of light shown in FIG. 4 is not especially suited to any particular application, it is intended to direct light into an angular range that can be intercepted by a secondary optic e.g., reflector 202. The goal is not fully achieved in that the angular distribution of light produced by the primary optic 302 covers a range that extends from zero polar angle and therefore all of the light cannot be intercepted by the reflector 202.
Another presently manufactured commercial optic 502 for LEDs is shown in FIG. 5. In use, an LED (not shown) will be located in a bottom recess 504. This optic 502 is one form of “secondary” optic. A LED with or without the primary optic 302 attached can be used. If used the primary optic will fit inside the bottom recess 504. The secondary optic 502 is made from optical grade acrylic (PMMA) and is completely transparent with no reflective coatings. The optic 502 includes a TIR (Total Internal Reflection) parabolic surface 506 which collects a first portion of light emitted by the LED, and a convex lens surface 508 which collects a remaining portion of the light. Both surfaces 506, 508 are intended to collimate light. As might be expected in actuality the light is distributed in a Gaussian-like angular distribution over a certain angular range which is variously reported as 5 degrees and 10 degrees. The former value may be a FWHM value, and the actual value will vary depending on the exact LED that is used. This design is only useful for a fairly narrow range of specialized applications that require a far-field highly collimated LED spotlight. FIG. 6 shows an angular distribution of light produced by this type of optic. As shown the angular distribution is Gaussian-like not uniform.
In order to get a broader angular distribution of light some form of surface relief pattern can be added to a top surface 510 of the optic 502 which is planar as shown in FIG. 5. Alternatively, the surface relief pattern can be formed on a “tertiary” optic that is attached to the top surface 510. One type of surface relief pattern-concentric rings of convolutions is shown in a plan view in FIG. 7 and in a broken-out sectional elevation view in FIG. 8. Another type of surface relief pattern—an array of lenslets is shown in a plan view in FIG. 9 and in a broken-out sectional elevation view in FIG. 10. FIGS. 11 and 12 show light intensity distributions produced by commercial optics that have the same general design as shown in FIG. 5 but which have top surfaces with a surface relief pattern to broaden the angular distribution. The distribution shown in FIG. 11 is designated as having a 15 degree half-angle pattern and that shown in FIG. 12 a 25 degree half angle pattern.
In fact at 25 degrees such designs are coming up against a limit. The limitation is explained as follows. Given that the optic 502 with a flat surface 510 nearly collimates light to within a nominal 5 degree half angle, it can be inferred that light is incident at the surface 510 at about 5/n degrees, where n is the index of refraction of the optic, which for the sake of the following can be considered zero i.e., collimated. In order to create broader distributions of light, some relief pattern as discussed above is added to the top surface 510. Portions of the relief pattern will be tilted relative to the light rays incident from below, and will therefore refract light out at larger angles than would the flat top surface 510. However, at about 25 degrees, depending on how much light loss will be tolerated a limited is reached—in particular the transmittance of the surface starts to decline rapidly. In this connection it is to be noted that according to Snell's law in order deflect light at a particular angle, say 25 degrees, the angle of incidence on the surface must be considerably larger than 25 degrees. FIG. 13, includes a plot 1302 that represents transmittance versus deflection angle when passing from a medium of index 1.6 (a typical value for visible light optics) into air. The X-axis in FIG. 13 is in radians. At the polar angle of 0.44 which is approximately equal to 25 degrees, the transmittance curve 1302 is already into a decline. The transmittance shown by plot 1302 is better than attained in practice because, at least, it does not take into account reflection losses experienced when light passes into the optic 502 through the bottom recess 504 of the optic. This is evidenced by reports of 90% and 85% efficiency for collimating versions of the optic as shown in FIG. 5, not 96% which is the starting value of plot 1302. In contrast, plot 1304 which applies to illumination lenses according to certain embodiments of the invention accounts for losses at both of two lens surfaces.
FIG. 14 shows another type of optic 1400 that is useful for illumination. This optic includes a saw tooth TIR section 1402 and a central lens portion 1404. The optic 1400 can collect a full hemisphere of emission from a source and forms an illumination pattern with a half-angle divergence (polar angle) about 30 degrees. This lens is disclosed in U.S. Pat. No. 5,577,492. For this type of optic there will be some loss of light from the intended distribution at the corners of the saw tooth pattern, which in practice may not be perfectly sharp due to manufacturing limitations. Additionally, due to its complex shape the cost of machining and polishing molds for injection molding is expected to be high. Additionally the '492 patent does address controlling the distribution of light within angular limits of the beams formed. The optic 1400 is already broad relative to its height. If an attempt were made to broaden the polar angle range of the illumination pattern, the TIR surfaces 1404 would have to be angled at larger angles, making the optic even broader-perhaps impractically broad